In cathode sputtering, particles are sputtered from a target and deposited onto a wafer mounted opposite the target. While the aggregate travel path of all the sputtered particles is generally direct, i.e., perpendicular from a planar target to a planar surface of an oppositely mounted wafer, a substantial number of particles travel on flight paths which are non-perpendicular. These sputtered particles traversing angled flight paths may adversely affect wafer coverage.
For example, instead of contacting and completely covering the bottom surfaces of vias or other features of a wafer, as would occur with sputtered particles taking perpendicular flight paths, sputtered particles traversing non-perpendicular angles are much more likely to deposit and accumulate on the side walls of features. The accumulation of sputtered material on the side walls of a feature prematurely closes off access to the bottom surface of the feature, before the bottom surface is entirely coated. As a result, unfilled voids are produced in the feature. This undesired closing off of access to the bottom surface of a feature is referred to as "necking" or overhang.
As shown in U.S. Pat. No. 4,724,060, it is known to position a collimator between a target and a wafer to intercept sputtered particles which traverse undesired angled flight paths toward the wafer. By intercepting particles which vary from a perpendicular flight path by more than a predetermined angle, more effective filling of the features of a wafer is assured. In short, a collimator provides control over the directionality of the flight paths traversed by sputtered particles which ultimately deposit on a wafer.
A typical collimator is grid-like in structure, with a plurality of substantially parallel passages. In cross section, the passages may be rectangular, circular, hexagonal, octagonal or any one of a number of other shapes. The parallel passages permit sputtered particles to travel therethrough in a direct, perpendicular flight path from the target to the wafer. Sputtered particles which traverse flight paths oriented at angles which vary only slightly from perpendicular will also pass through the passages Sputtered particles which traverse flight paths oriented at angles which vary substantially from perpendicular will be intercepted by the collimator.
The shapes of the passages of the collimator, sometimes referred to as the "unit cells", determine the cut-off angle, or critical angle, of flight whereby sputtered particles are either intercepted or permitted to pass therethrough. More particularly, the aspect ratio, i.e., the ratio of cell length to cell width, of each unit cell determines the critical angle. Cell length refers to the dimension measured from the target to the wafer along the shortest path. Cell width is measured perpendicular to cell length, at the greatest width along the length of the cell.
Assuming a generally planar target, a generally planar wafer and a generally planar collimator located therebetween, a direct path from the target through the collimator to the wafer will be referred to as a perpendicular, or 90.degree. flight path. All sputtered particles which traverse a 90.degree. flight path will pass through the collimator, except for a relatively small number which will collide with the leading edges of the collimator walls. For sputtered particles traversing a flight path at an angle other than 90.degree., those traversing flight paths which vary from 90.degree. by more than the critical angle will be intercepted by the collimator.
For a collimator having unit cells with an aspect ratio of one, i.e., a length of one unit measured along the 90.degree. flight path and a width of one unit, the critical angle is 45.degree.. Sputtered particles traversing a flight path which varies from 90.degree. by more than 45.degree., i.e., a flight path of less than 45.degree. or greater than 135.degree. with respect to the target surface, will be intercepted by the collimator. Sputtered particles which traverse a flight path greater than 45.degree. and less than 135.degree., again with respect to the surface of the target, will not be intercepted. As the aspect ratio of the unit cells of a collimator increases, the critical angle decreases and more of the particles traversing angled flight paths are intercepted.
For a collimator with unit cells that are circular in cross-sectional shape, the aspect ratio, and hence the critical angle, is the same regardless of the angular orientation of the collimator with respect to the target and the wafer. This is true because the width of the unit cells is always equal to the diameter. However, if the unit cells are rectangular in cross-sectional shape, or perhaps hexagonal, the aspect ratio measured from corner to corner will differ from the aspect ratio measured between parallel surfaces.
In the context of this application, the aspect ratio of a unit cell which is non-circular in cross-section is considered to be the lowest possible value, i.e., the greatest possible width along the length of the unit cell. This means that the critical angle of a unit cell is the greatest possible angle of variation in flight path from 90.degree. which a sputtered particle may traverse and still pass through the collimator. For example, for a unit cell which is rectangular in cross-section and uniform in transverse cross sectional shape along its length, the width dimension used to measure the aspect ratio will be the corner to corner dimension, thereby providing the lowest possible aspect ratio and the greatest possible critical angle of flight a sputtered particle may traverse and still pass through this unit cell.
During sputtering, intercepted particles continuously accumulate on the exposed surfaces of the walls which define the unit cells of the collimator. This accumulation reduces the effective open area of the unit cells. In this application, the term effective open area with respect to a unit cell refers to the smallest transverse cross sectional surface area of open space through the unit cell along its entire length. To deposit on the wafer, a sputtered particle must pass through the effective open area of one of the unit cells of the collimator.
Because the lengths of the unit cells do not change, the accumulation of intercepted particles increases the aspect ratio and decreases the critical angles of the unit cells. Therefore, of those sputtered particles traversing flight paths which are not 90.degree., the relative proportion of those that are intercepted will increase with respect to those that pass through, as more accumulation occurs. Moreover, this increase in the proportion of particles which are intercepted is continuous during sputtering, because of the continuous decrease in effective open area caused by accumulation. This effect is further compounded by unit cell shapes which are non-circular, because the rate of accumulation relates, to some degree, on the narrowest width of the unit cells.
The continuous reduction in the effective open area of the unit cells also has another effect. Of the sputtered particles which traverse 90.degree. flight paths, the relative proportion of those that pass through the collimator will also decrease with respect to those that are intercepted. This is due to the decrease in open area compared to blocked or impeded area along the length of a passage through the collimator.
Thus, particle accumulation causes effective open area reduction in the unit cells, and this reduction has two adverse effects. Both of these effects are made more acute by the fact that the greatest concentration of intercepted particle accumulation occurs adjacent the target side, or entrance, to the unit cells. The rate at which unit cell effective open area decreases is relatively high, and it is continuous.
As a result of both of these effects, the deposition rate of sputtered particles onto the wafer eventually decreases to such a low value that further use is ineffective, and the collimator must be replaced or removed and cleaned. Replacement and/or cleaning of a collimator requires shutting down the wafer processing apparatus and breaking the vacuum to the chamber in which the target and the wafer are housed. Every maintenance-required shut down and restart of a wafer processing apparatus represents lost time in wafer processing, or reduced wafer throughput.
Presently, typical collimators must be replaced after the sputter coating of about 300 wafers. This number is considered by applicant to be too low, or, in another manner of reference, the frequency of collimator replacement and/or cleaning is considered too high to achieve desired throughput capability.
It is an object of this invention to provide a collimator with an extended lifetime compared to prior collimators, thereby to increase the number of wafers which may be sputter coated prior to replacement and/or cleaning of the collimator.
It is another object of the invention to provide an extended lifetime collimator which enhances throughput capability in the sputter coating of wafers.